An outer surface, which is supported by support rolls, of a cast slab solidifies first upon cast in continuous casting equipment. Thus, the center of the casting thickness (in the cast slab thickness direction) solidifies last. In addition, when molten steel solidifies, volume shrinkage of 3 to 4% occurs. Therefor, a micro-cavity that is called central porosity inevitably forms in the cast slab center which solidifies last. This central porosity remains even after rolling, and can be detected under ultrasonic testing in the stage of a thick steel plate. Internal flaws caused by this central porosity are harmful especially according to a use of a nuclear reactor, a boiler, a pressure vessel or the like. Thus, to reduce the central porosity volume in a cast slab is conventionally carried out.
Segregation is easy to form in addition to porosity in the cast slab center which solidifies last. Especially, it is difficult to reduce both the porosity volume and segregation at the same time in a slab having granular equiaxed crystals in its center. The following are some reasons considered: (1) segregation is easy to form around granular equiaxed crystals; (2) if granular equiaxed crystals move at the end of solidification, a segregation portion also moves along with the granular equiaxed crystals, and segregating elements are easy to gather at a portion surrounded by a plurality of gathering granular equiaxed crystals, which makes segregation easy to be large; and (3) porosity is easy to form at a portion surrounded by segregation that forms around granular equiaxed crystals. Therefore, so far, porosity and segregation have been tried to be remedied by making columnar crystals with which both porosity and segregation are easier to be reduced at the same time than with granular equiaxed crystals, easy to grow.
In a case where the central porosity volume is reduced by heavily rolling a cast slab in the later process, conventionally, it is necessary to carry out heavy rolling of no less than 0.7 in shape factor γ in the later process in order to reduce central porosity of a conventional cast slab of 230 to 380 mm in thickness (casting thickness) D to a level with passing ultrasonic testing at the stage of a thick steel plate. To carry out such rolling, the cast slab has to be heated at a high temperature to 1250° C. or more, which requires a high cost. The shape factor γ is an indicator used for showing a degree of rolling, and a value thereof is defined by the formula: contact arc length of a reduction roll with a steel plate/mean plate thickness=(R(h0−h1))0.5/(0.5(h0+h1)), where R is a roll diameter (mm), h0 is entry side plate thickness (mm) and h1 is delivery side plate thickness (mm).
In order to reduce the amount of forming central porosity at the casting stage, for example, Patent Literature 1 discloses the art that after perfect solidification of a slab when the surface temperature of the slab is 700 to 1000° C., the slab is sandwiched between upper and lower rolls each having a projecting portion at the center and is subjected to rolling reduction to be crushed, to reduce central porosity.
Patent Literature 2 discloses the art that before completing the solidification after bulging the cast slab into 10 mm or more, the center part of the width is subjected to reduction, and successively, the vicinity of either edge part is subjected to reduction, so that the solidified interfaces are press-stuck.
Patent Literature 3 discloses the art that the center part in the thickness of the cast slab is subjected to reduction in the continuous casting equipment in the condition of 1400° C. to the solidified temperature thereat.